In the past, the majority of dry type transformers have been impregnated with or encapsulated in resinous materials dissolved in solvents, particularly aromatic solvents. The solvents in these impregnants or encapsulants serve a multitude of purposes. For example, they dilute the resin solids or high viscosity liquids in order to provide a system viscosity which will assure complete impregnation or encapsulation of the transformer coil, and other insulating materials. Further, the solvents serve to separate or space the solvent-resin-activator system in order to minimize molecular contact reactions which would tend both to increase viscosity and shorten pot life.
However, the inclusion of the solvent in the impregnant or encapsulant also tended to increase processing cycles, increase energy costs, as energy was required to vaporize the solvent. and add to resin losses in the processing operation. Further, as the solvents employed were frequently vented to the atmosphere as they were evaporated from the transformers, they tended to significantly increase atmospheric pollution. Further, these solvents provided fire hazards, acted as cure retardants until they were evaporated from the product, and generated additional costs because of the additional safety provisions which were required.
While solventless resin systems had been previously developed for the uses set forth herein, and while they are adequate for most purposes, they do present certain problems. For example, the silicone materials remain extremely expensive while the polyesters, either employed as such, or generated because of reaction between various components in a resin system, have a degree of hydrolytic instability.